Don't request sweep thread during GC.start immediate

Author: luke-gruber

gc/default/default.c

@@ -5437,6 +5437,10 @@ gc_sweep_start(rb_objspace_t *objspace)
     bool do_signal = false;
     if (objspace->sweep_thread &&
             !objspace->flags.during_compacting &&
+            /* A major immediate GC sweeps the rest synchronously (gc_sweep_rest)
+             * right after this, so handing work to the background thread only adds
+             * lock contention with no chance to overlap. Keep it on the FG. */
+            !(is_full_marking(objspace) && objspace->flags.immediate_sweep) &&
             !(objspace->hook_events & RUBY_INTERNAL_EVENT_FREEOBJ)) {
         psweep_debug(-1, "[gc] gc_sweep_start: requesting sweep thread\n");
         objspace->use_background_sweep_thread = true;
@@ -5962,23 +5966,23 @@ background_sweep_done_p(rb_objspace_t *objspace)
 static void
 gc_sweep_rest(rb_objspace_t *objspace)
 {
-#if USE_PARALLEL_SWEEP
-    bool do_signal = false;
-    sweep_lock_lock(objspace);
-    {
-        objspace->sweep_rest = true; // reset to false in `gc_sweeping_exit`
-        if (!background_sweep_done_p(objspace)) {
-            if (objspace->use_background_sweep_thread && !objspace->sweep_thread_sweeping && !objspace->sweep_thread_sweep_requested) {
-                do_signal = true;
-                objspace->sweep_thread_sweep_requested = true;
-            }
-        }
-    }
-    sweep_lock_unlock(objspace);
-    if (do_signal) {
-        rb_native_cond_signal(&objspace->sweep_cond);
-    }
-#endif
+/*#if USE_PARALLEL_SWEEP*/
+    /*bool do_signal = false;*/
+    /*sweep_lock_lock(objspace);*/
+    /*{*/
+        /*objspace->sweep_rest = true; // reset to false in `gc_sweeping_exit`*/
+        /*if (!background_sweep_done_p(objspace)) {*/
+            /*if (objspace->use_background_sweep_thread && !objspace->sweep_thread_sweeping && !objspace->sweep_thread_sweep_requested) {*/
+                /*do_signal = true;*/
+                /*objspace->sweep_thread_sweep_requested = true;*/
+            /*}*/
+        /*}*/
+    /*}*/
+    /*sweep_lock_unlock(objspace);*/
+    /*if (do_signal) {*/
+        /*rb_native_cond_signal(&objspace->sweep_cond);*/
+    /*}*/
+/*#endif*/
     for (int i = 0; i < HEAP_COUNT; i++) {
         rb_heap_t *heap = &heaps[i];
         while (!heap_is_sweep_done(objspace, heap)) {